Improve pathfinder support for multiple docks

The pathfinder can now find docks other than the Manhattan closest one.

Based on Cirdan commits:
afc7969c13d7ca59afe4dae4bf88122ba8d27df2
4067190dedcd3e5f668ea4f49b1dd8dfed10b2a7
d9be6b712d2ae4fb1b5ae844dde4919dd24c4fb2

src/station_base.h

@@ -510,6 +510,8 @@ public:
 		return IsAirportTile(tile) && GetStationIndex(tile) == this->index;
 	}
 
+	bool IsDockingTile(TileIndex tile) const;
+
 	/* virtual */ uint32 GetNewGRFVariable(const ResolverObject &object, byte variable, byte parameter, bool *available) const;
 
 	/* virtual */ void GetTileArea(TileArea *ta, StationType type) const;